Sensitive
seismographs are the principal tool of scientists who study
earthquakes. Thousands of seismograph stations are in operation throughout
the world, and instruments have been transported to the Moon, Mars, and
Venus. Fundamentally, a seismograph is a simple pendulum. When the ground
shakes, the base and frame of the instrument move with it, but intertia
keeps the pendulum bob in place. It will then appear to move, relative to
the shaking ground. As it moves it records the pendulum displacements as
they change with time, tracing out a record called a seismogram.One seismograph station, having three different pendulums sensitive to the north-south, east-west, and vertical motions of the ground, will record seismograms that allow scientists to estimate the distance, direction, Richter Magnitude, and type of faulting of the earthquake. Seismologists use networks of seismograph stations to determine the location of an earthquake, and better estimate its other parameters. It is often revealing to examine seismograms recorded at a range of distances from an earthquake:
On this example it is obvious that seismic waves take more time to arrive at stations that are farther away. The average velocity of the wave is just the slope of the line connecting arrivals, or the change in distance divided by the change in time. Variations in such slopes reveal variations in the seismic velocities of rocks. Note the secondary S-wave arrivals that have larger amplitudes than the first P waves, and connect at a smaller slope.
While the actual frequencies of seismic waves are below the range of human hearing, it is possible to speed up a recorded seismogram to hear it. You can click on this earthquake recording to hear a seismogram from the 1992 Landers earthquake in southern California, recorded near Mammoth Lakes in an active volcanic caldera by the USGS. The original record, 800 seconds long, has been speeded up 80 times so that you hear it all within 10 seconds.
Listen to more earthquakes with:
- John Louie's The Sound of Seismic (MP3s)
- Andy Michael and Dennis Ross's Listening to Earthquakes (WAVs)
Locating Earthquakes
The pricipal use of seismograph networks is to locate earthquakes. Although it is possible to infer a general location for an event from the records of a single station, it is most accurate to use three or more stations. Locating the source of any earthquake is important, of course, in assessing the damage that the event may have caused, and in relating the earthquake to its geologic setting.
Given a single seismic station, the seismogram records will yield a
measurement of the S-P time, and thus the distance between the station and
the event. Multiply the seconds of S-P time by 8 km/s for the kilometers
of distance. Drawing a circle on a map around the station's location, with
a radius equal to the distance, shows all possible locations for the
event. With the S-P time from a second station, the circle around that
station will narrow the possible locations down to two points. It is only
with a third station's S-P time that you can draw a third circle that
should identify which of the two previous possible points is the real one:
This example uses stations in Boston, Edinborough, and Manaus. With the distances shown, all three circles can intersect only at a single point on the Mid-Atlantic Ridge spreading center.
Information provided by: http://www.seismo.unr.edu